Is the reactivity of calcium carbonate fully understood?

Abstract

In the Earth's surface environment, calcium carbonates are abundant (Sekkal and Zaoui, 2013), in particular in carbonate-rich sedimentary rocks, coral reefs, stalactites, and stalagmites.CaCO3 is also the largest long-term stable sink for CO2 in the global carbon cycle. Moreover, dissolved calcium and carbonate can be found, for instance, dissolved in natural waters (rivers and oceans), and are the main building blocks of many biominerals. Many organisms including primary producers such as coccolithophorids and cyanobacteria, as well as secondary producers foraminifera and diverse animals (bivalves, pteropods), process CaCO3 to create the required polymorph for their biominerals in a (more or less) controlled manner (Endo et al., 2018;Meldrum and Cölfen, 2008;Morse et al., 2007). This formation is called biomineralization and can lead to surprising structures and polymorphs due to the modification of the environment such that thermodynamic conditions for CaCO3 precipitation are optimized (Blue et al., 2017;Von Euw et al., 2017) The most common way this mineral is preserved and found in nature is in carbonate-rich sandstones, marble, or chalk and limestone that are composed of the remains of shells and skeletons of sea organisms and/or calcium carbonate matrix. Such rocks host roughly half of the earth's oil and gas reserves (Liteanu et al., 2013;Roehl and Choquette, 1990). Next to its natural role on Earth, calcium carbonates have been reported at the surface of Mars, using remote sensing techniques and studying weathering profiles (Bultel et al., 2019;Wray et al., 2016). These observations suggest the existence of carbonic acids dissolved in liquid water, more than 3.7 billion years ago, that reacted and changed Mars' surface, hinting towards an environment suitable for some form of life (Bultel et al., 2019). Furthermore, research on calcium carbonate formation is important in a broad range of (applied) research and engineering fields and industry. From money-driven (e.g. oil industry (Olajire, 2015), paper industry (Vashistha et al., 2021), geothermal heat extraction (Pandey et PolymorphsCalcium carbonate has six different polymorphs: three forms are crystalline, two are hydrated phases, and one is non-crystalline.Amongst the crystalline phases, firstly, calcite is thermodynamically the most stable polymorph at ambient temperature and pressure and therefore most commonly found in nature, in the Earth's surface environment. Calcite is hexagonal, a trigonal system, and its space group is 𝑅3 𝑐 (Rachlin et al., 1992). Ca 2+ is coordinated with six carbonate oxygens and the structure is built up of alternating calcium and carbonate groups, the carbonate's plane perpendicular and parallel to the c-axis. Its most stable cleavage plane is {101 4}, which results in a perfect rhombohedral shape (Rachlin et al., 1992). Secondly, aragonite is the second most commonly found polymorph of calcium carbonate and is less stable than calcite at ambient conditions.The crystallization of calcium carbonate has been studied for deca...